In the early embryo, the cranial base, the mandibular condyle and other dental support tissue are composed of cartilage. As development progresses, most of these cartilages undergo maturation, hypertrophy and endochondral ossification and are replaced by bone. The remaining cartilages persist throughout life at specific skeletal sites, such as the condyle head and the articulating surface between cranial base and vertebrae. The mechanisms by which embryonic cartilages undertake these alternative developmental pathways and give rise to permanent cartilages and transient endochondral cartilages are largely unknown. In this proposal we aim to demonstrate that the transforming growth factor beta isoforms (TGFbetas) have distinct effects and actions at different chondrocyte developmental stages. Specificity of TGFbeta action would depend upon: (a) the complement of TGFbeta isoforms produced; (b) the ability of the individual chondrocyte population to activate latent forms of TGFbeta; (c) the class and number of receptors present on the cell surface and hence, the responsiveness of the cells to TGFbeta; and (d) the availability of TGFbeta stored on or trapped within the extracellular matrix compartments. Preliminary studies indicate that developmentally distinct populations of chondrocytes produce large quantities of latent TGFbeta in serum-free agarose cultures, and that co-cultures of different chondrocyte populations result in alterations of the phenotype. The concerted experimental efforts described in this proposal will provide much needed basic information on the biology of TGFbeta in skeletogenesis. The results will shed light on how this potent factor can exert distinct effects at different skeletal sites, and in so doing, contributes to normal spatial and temporal development of cartilages and bones in the embryo and early postnatal animal. These results may allow one to envision how skeletal defects may originate from alterations in TGFbeta-dependent pathways.
